TYPO3 vulnerable to Improper Access Control Persisting File Abstraction Layer Entities via Data Handler

TYPO3 is an open source PHP based web content management system released under the GNU GPL. In affected versions of TYPO3 entities of the File Abstraction Layer (FAL) could be persisted directly via `DataHandler`. This allowed attackers to reference files in the fallback storage directly and retrieve their file names and contents. The fallback storage ("zero-storage") is used as a backward compatibility layer for files located outside properly configured file storages and within the public web root directory. Exploiting this vulnerability requires a valid backend user account. Users are advised to update to TYPO3 version 8.7.57 ELTS, 9.5.46 ELTS, 10.4.43 ELTS, 11.5.35 LTS, 12.4.11 LTS, or 13.0.1 which fix the problem described. When persisting entities of the File Abstraction Layer directly via DataHandler, `sys_file` entities are now denied by default, and `sys_file_reference` & `sys_file_metadata` entities are not permitted to reference files in the fallback storage anymore. When importing data from secure origins, this must be explicitly enabled in the corresponding DataHandler instance by using `$dataHandler->isImporting = true;`.